Merits of directed paths
Having directed play or a railroad narrative experience is sometimes required to keep players on track. As much as people love to say that freedom is the best thing for any player, it often isn’t. Everyone needs a goal to direct their attention and energy.
The problem with the directed play is that if it continues for the entirety of the experience then it can become increasingly heavy-handed, especially in certain formats and environments. As the further you get into a linear narrative, the more constricted the player will feel, due to diminishing paths and the taking away of choices from them.
Eventually, the player will be a spectator, the narrative will continue despite the player’s choices and thus they lose an intrinsic sense of agency. They may still feel some agency, but it will be a vicarious feeling like the sensation felt for a character in a movie.
It is important when delivering adventure experiences that the players are taking part in the story and are able to “tell” their own stories as part of a larger narrative that you as the game master are facilitating. The telling of the story is that once they are established with the narrative world, they drive the momentum forward with their own actions. And that if they see opportunities for choice, that these are possible for them to undertake.
Merits of milestones
Milestones are the crux points in a narrative. A good metaphor is if you imagine the narrative adventure as a river, the end goal is at the end of the river (downstream). Players can move however they want in the river, go straight downstream, swim a little upstream (against the current) or swim side to side. But when they do go with the current downstream, they’ll come across some rocky outcroppings (milestones) which they must get past to continue.
If you choose a more flexible, sandbox narrative, then having milestones allows for some direct play moments at certain time points or overall plot points. But leaves the player to choose how and what to do in the preceding and following moments. Maintaining a more self-determining playstyle.
The benefit of milestones over fully directed play, therefore, is the ability to offer (the illusion of) choice to the player. Players have the option to go whichever direction they want within the narrative, you as the game master therefore also have the option to place challenges in their path wherever you want, as there are no real linear limitations.
You can think of it as Schrodinger’s Cat until players choose a direction, the possible challenge they may encounter is both about to happen and not happen at the same time. It is left to you as a game master to choose which it is and how difficult it is, or if it occurs at all.
The problem with fully using this method is that as the game master though you will need to be very flexible and very well prepared for any eventuality. You will also need to be in tune with what your players are doing and what their expectations are. Even with the illusion of choice, being too open can quickly become overwhelming for you and eventually also for the players.